Plurally charged ion beam generation method



June 9, 1964 J. A. WEINMAN 3,136,908

PLURALLY CHARGED ION BEAM GENERATION METHOD Filed July 28, 1960 KEY ONEUTRAL ATOMS MULTIFLY CHARGED INVENTOR Posmvs IONS SINGLY CHARGED JAMESA. WEINMAN PosmvE IONS ATTORNEY United States Patent Ofifice 3,136,908Patented June 9, 1964 3,136,908 PLURALLY CHARGED ION BEAM GENERATIONMETHOD James Adolf Weinman, Downers Grove, Ill. (2506 Kendall Ave,Madison, Wis.) Filed July 28, 1960, Ser. No. 45,983 5 Claims. (Cl.313-63) This invention relates to nuclear accelerators, and moreparticularly relates to a machine and a method for providing a highintensity beam of plurally charged ions of atomic number of from 2 toapproximately 20 available for acceleration at a high voltage terminalof an accelerator.

The DC. acceleration of plurally charged positive ions.

has conventionally required that an ion source, ion focusing equipmentand other supplementary ion beam generation equipment be installedwithin a high voltage terminal contained within a sealed and pressurizedelectrostatic or Cockroft Walton accelerator enclosure. The presence ofsuch equipment within an enclosed high voltage terminal complicates theconstruction of an accelerator and renders maintenance of the ion sourcedifiicult. It also limits the type of ions to be accelerated to thosewhich are readily produced by a compact source.

A source of positive ions such as is usually installed in the highvoltage terminal of an electrostatic accelerator provides positive ionsfrom an electric discharge in a tenuous atomic gas, the ions of whichone wishes to accelerate. The gas usedto obtain plurally chargedparticles may most conveniently be helium, but one is not restricted tothe use of this gas. Design of such sources are manifold and are wellknown to those concerned with the art (D. Kamke Encylopedia of Physics,vol. 33, 1956, p. 36, et seq). This invention is not restricted to useof any particular design of poistive ion source or DC. accelerator, butproposes to place a source at a location remote from an electrostaticaccelerator high voltage terminal.

This invention provides a method and a machine for generating a positiveion beam of plurally charged particles at an accelerator terminal from abeam of fast (nonthermal) neutral atoms produced at a source outside theaccelerator enclosure. Generating the neutral particle beam outside theterminal enclosure greatly facilitates the adjustment and operation ofthe source mechanism and makes possible the production of ion beamswhich could not readily be produced subject to the space and powerrestrictions within an accelerator terminal.

It is an object of this invention to provide a high intensity beam ofplurally charged ions of elemental gases having atomic numbers of from 2to approximately 20.

Other objects will become apparent from the drawings and from thefollowing detailed description in which it is intended to illustrate theapplicability of the invention without thereby limiting its scope toless than that of all equivalents which will be apparent to one skilledin the art. In the drawings like reference numerals refer to like partsand:

The single figure is a schematic elevation of an electrostaticaccelerator of this invention.

In the figure is shown electrostatic accelerator 12. Accelerator 12comprises pressurized enclosure 11 containing positively charged highvoltage terminal charging belt 15, equi-potential rings 13, andevacuated tube 22 disposed axially therewithin. Terminal 16 may becharged to a positive potential of several million volts with respect toterminal 17 by electrostatic charge transfer from belt 15, and may beelectrically insulated from enclosure 11, of which terminal 17 is aportion, by insulators between equi-potential rings 13 in a mannerconventional to electrostatic accelerators. Another method of achievingthe high voltage on the terminal is by an electronic voltage multiplyingcircuit first applied to this use by Cockroft and Walton. Theconstruction of a high voltage terminal and the means for charging andinsulating the terminal form no part of this invention.

Ion source 21 is shown schematically and may include a source ofsuitable gas and a gas purifying device. Purification may beaccomplished in conventional fashion by passing the gas through a liquidair trap to remove contaminants before it is introduced into accelerator12.

Purified molecular gas of atomic number 2 or greater may be singly ormultiply ionized in an electrical discharge of appropriate design. Meansfor producing positive ions are well known to those versed in the artand any suitable such means may be utilized.

Singly and multiply charged positive ions may be extracted from ionsource 21 of FIGURE 1 by electrode 26. Electrode 26 may be negativelycharged to between and 2000 kilovolts and preferably to between 500 to1000 kilovolts with respect to ion source 21, if positive helium ionsare to be used. Voltage of this magnitude can readily be obtained eitherby an electrostatic generator, or by a voltage multiplying electroniccircuit. The positive ions (multiply and singly charged) from source 21are then passed through evacuated tube 22. Either the positive ionsource 21 may be at ground potential and target 27 be negatively biasedwith respect thereto, or target 27 may be at ground potential and theion source 21 be positively biased with respect to target 27 (notillustrated for simplicity), or the two components may be biased toopposite polarities with equal effect to provide a suitable potentialbetween the two (also not illustrated for simplicity). It may bedesirable to provide between ion source 21 and electrode 26 a suitablemeans for focusing the ion beam such as an electrostatic lens (notshown).

Within electrode 26 is charge exchange target 27. Target 27 ispreferably a canal in which a quantity of gaseous matter (e.g. hydrogen,nitrogen, argon, or oxygen) is disposed in the path of travel of theparticle beam, such matter having a concentration substantially greaterthan matter in adjacent areas through which the beam passes. It may alsoconsist of a thin metal or carbon foil disposed across the path of thebeam. Both gaseous and foil charge exchange targets are known in the artand form no part of this invention. The ions, upon passing throughtarget 27 pick up electrons and emerge therefrom as neutral particles,i.e. a beam of fast (non-thermal) atoms. The charged particles (bothmultiply and singly charged) that then remain in the beam may then, ifdesired, be deflected by charged collector ring or magnet 31 and beremoved from the beam. The uncharged atoms which comprise the operableportion of the beam can then drift with the energy previously givenherein, i.e. (in the case of helium isotopes, for example) from 100 to2000 kilo electron volts, and preferably from 500 to 1000 kilo electronvolts, into the electric field of terminal 16 which is maintained at ahigh positive potential, e.g. several million volts, without beingrepelled. Thereupon the atoms can pass through target 28.

Target 28 may be a quantity of matter similar to target 27, e.g. a metalor carbon foil or a gas filled duct. Gas is the preferred targetmaterial and can most conveniently be oxygen or nitrogen, although othergas might also be used. Some of the atoms in the beam upon passingthrough target 28 in terminal 16 are ionized by collisions with thematter of target 28 and consequently lose their electrons. Some may loseone electron to form singly charged ions and some lose two or moreelectrons to form multiply charged ions. The ions that are formed maythen be accelerated to ground potential at terminal 17 in a single mainstage of acceleration in conventional.

such emergent ions which are multiply charged; since their The ionsemerge with an energy proportional to chargewill be twice as great ormore than twice as great as the charge of singly charged ions, theirenergy on Usually the emergence will be at least twice as great. beam ofmultiply charged ions which emerges from: terminal'17 will also containsingly charged ions and neutral.

atoms. It is desirable, for the purposes of the invention,

a that a significantfractionof the atoms which pass through terminal 16are stripped of two -or more electrons so that theemergent beam at 17contains a significant fraction of multiply charged ions. Useful andvaluable results are obtained if at least .002% of the particles whichemerge at 17 are multiply charged ionsso that a significant fraction orportion is to beconsiclercd to be .002% in the case of this invention.

charged ions may then be provided by passing the beam through a magneticfield, which may be created in any suitable manner, such as by magnet35, whereupon the beam of multiply charged ions is bent so that itemerges as indicated by arrow 36; the paths of singly charged ions arebent less with the result that they strike the wall of the tube; thepaths of neutral atoms are of .course. not

changed by the magnetic field ancl they ofcourse likewise "30 aintensity beam of multiply charged. positive ions having an;

strike the wall of the tube -at 35. The term thermal velocity as it isused velocities not greater than 2x 10 cm./sec.

herein denotes fFast (nonthermal) atoms or ions are those having avelocity of a v I 7 atoms having an atomic number of "from 210 20drifting .toward a terminal having a high positive potential of 10-cm./sec. or greater; a I

The following chart shows, as anexample, the relationship between someenergies and velocities for two isotopes of helium for use in'theaccelerator'ofthis inven- While certain modifications and embodiments ofthe invention have been described, it is of course to be then removingmore than one electron from a significant.

fraction of each of: said .atoms at'said first terminal to form multiplychargedvpositiveions, then only once accelcrating said ions toward andthrough said second terminal f 4 a tensity beam of multiply chargedpositive ions having an atomic number of from 2 to 20, comprising thesteps of providing a quantity of thermal neutral atoms-having an atomicnumber of from 2 to 20, io nizing said atoms tot r-positive ions,accelerating said ions across a first voltage differential to provide abeam of fast (non'-thermal)'ions,

then adding electrons to said ions tdprovide" a beam of fast(non-thermal) neutral atoms drifting toward a first terminal having ahigher positive potential of several million volts with respect .to asecondterminal than the potential difference across said first'voltagedifferential,

with respect to which'said first terminal hasllsaid high positivepotential to provide a high energy and high intensity :beam of multiplycharged 'fromsaid second. terminal. r p 20 7 A beam which containssubstantially only multiply 7 positive ions: emerging ybeam to add saidelectrons thereto and passing said beam of drifting fast (non-thermal)neutral atoms through a quantity of matter present at said firstterminal in greater concentration than in' adjacent portions of. saidpath of said beam 'to remove more than one electron from asignificantfraction of-each o f said atoms. I

3; The method of providing-a'high energy and high atomic number" of from2 to 20, .comp'rising the steps of firstproviding a beamofjfastnon-thermal) neutral several million volts with respect to a'sec'ondterminal,

tion. The superscripts denote the atomic weight of the positive ion.

CmJsec. 100 kev 1000 kev 1 2000 kev.

' Ee 2.s 10 231x10 11x10. 7 He 2.2 10 6.9X108 9.7 10

then removing electrons from said neutral atoms at said first terminalto form multiply charged positive'ions, then accelerating said ionstoward and through said secondter-f minal with respect to which'saidfirst terminal has said i 7 "positive potential to provide a highenergy, and high intensity beam of multiply charged positiveions'emerging' from said second terminal, said potential of severalmillion'volt's adding more energy to saidions than'added to f providesaid fast neutral atoms.

4. The method-of claim 3 further, characterized by v passing said beamof drifting fast (non-thermal) neutral V 5. The method of claim 3characterized by at least a i significantly large proportion of saidpositive ions emerging from said second terminalbeing doubly chargedhelium ions.

understood that there are a great number of variations which willsuggest themselves to anyone familiar with the subject-matter thereofand his to be distinctly understood that this invention should not belimited except by "such limitations as are clearly imposed in theappended claims V V 1 I claim:

References Cited in the file ofthis" patent 7 Vi UNITED "STATES PATENTSoTHER REFERENCE-S7. j

' Bennett, July 2, 1940; Kuhnet al. 0m. 22 1940

1. THE METHOD OF PROVIDING A HIGH ENERGY HIGH INTENSITY BEAM OF MULTIPLYCHARGED POSITIVE IONS HAVING AN ATOMIC NUMBER OF FROM 2 TO 20,COMPRISING THE STEPS OF PROVIDING A QUANTITY OF THERMAL NEUTRAL ATOMSHAVING AN ATOMIC NUMBER OF FROM 2 TO 20, IONIZING SAID ATOMS TO POSITIVEIONS, ACCELERATING SAID IONS ACROSS A FIRST VOLTAGE DIFFERENTIAL TOPROVIDE A BEAM OF FAST (NON-THERMAL) IONS, THEN ADDING ELECTRONS TO SAIDIONS TO PROVIDE A BEAM OF FAST (NON-THERMAL) NETURAL ATOMS DRIFTINGTOWARD A FIRST TERMINAL HAVING A HIGHER POSITIVE POTENTIAL OF SEVERALMILLION VOLTS WITH RESPECT TO A SECOND TERMINAL THAN THE POTENTIALDIFFERECE ACROSS SAID FIRST VOLTAGE DIFFERENTIAL, THEN REMOVING MORETHAN ONE ELECTRON FROM A SIGNIFICANT FRACTION OF EACH OF SAID ATOMS ATSAID FIRST THERMINAL TO FORM MULTIPLY CHARGED POSITIVE IONS, THEN ONLYONCE ACCELERATING SAID IONS TOWARD AND THROUGH SAID SECOND TERMINAL